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Coasting external shock in wind medium: an origin for the X-ray plateau decay component in Swift GRB afterglows

The plateaus observed in about one half of the early X-ray afterglows are the most puzzling feature in gamma-ray bursts (GRBs) detected by Swift. By analyzing the temporal and spectral indices of a large X-ray plateau sample, we find that 55% can be explained by external, forward shock synchrotron emission produced by a relativistic ejecta coasting in a ρ~ r^{-2}, wind-like medium; no energy injection into the shock is needed. After the ejecta collects enough medium and transitions to the adiabatic, decelerating blastwave phase, it produces the post-plateau decay. For those bursts consistent with this model, we find an upper limit for the initial Lorentz factor of the ejecta, Γ_0 \leq 46 (ε_e/0.1)^{-0.24} (ε_B/0.01)^{0.17}; the isotropic equivalent total ejecta energy is E_{iso} ~ 10^{53} (ε_e/0.1)^{-1.3} (ε_B/0.01)^{-0.09} (t_b/10^4 s) erg, where ε_e and ε_B are the fractions of the total energy at the shock downstream that are carried by electrons and the magnetic field, respectively, and t_b is the end of the plateau. Our finding supports Wolf-Rayet stars as the progenitor stars of some GRBs. It raises intriguing questions about the origin of an intermediate-Γ_0 ejecta, which we speculate is connected to the GRB jet emergence from its host star. For the remaining 45% of the sample, the post-plateau decline is too rapid to be explained in the coasting-in-wind model, and energy injection appears to be required.